map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn] map f [x1, x2, ...] == [f x1, f x2, ...]
>>> map (+1) [1, 2, 3] [2,3,4]
toNonEmpty (map f xs) = map f (toNonEmpty xs)
map id = cat map (g . f) = map f >-> map g
>>> HashSet.map show (HashSet.fromList [1,2,3]) HashSet.fromList ["1","2","3"]